Agricultural combines are large machines that harvest, thresh, separate and clean agriculturally raised crops bearing grain. The clean grain obtained is stored in a grain tank arranged on the combine. The threshed straw is generally either chopped and spread out on the field across the width of the chopping mechanism, or diverted around the straw chopper and deposited unchopped in a swath in order to be subsequently taken up in a baler. The remaining crop residue at the rear outlet of the cleaning system, such as chaff and small pieces of straw, is distributed on the field by a chaff scattering device, or directed through the straw chopper and spread on the field.
The cleaning unit comprises a cleaning blower that draws in outside air and conducts the air from below against cleaning screens that are set into an oscillating motion so that heavy crop particles, more particularly grains, fall down through the cleaning screens, and impurities such as straw particles and chaff are blown away to the rear.
Therefore, a positive pressure is created inside the cleaning unit, i.e. there is an air pressure that is greater than the ambient pressure. In the prior art, the cleaning air stream is output into the surroundings to a relatively small degree through lateral housing openings of the combine, while the main part of the cleaning air stream is conveyed away by the straw chopper (cf. DE 196 24 733 A1 and DE 100 64 356 A1 for example), for which special air conveying means such as air baffles are provided on the straw chopper rotor (DE 10 2006 017 789 A1) or on the chopper blades (DE 10 2004 049 965 A1).
It has also been proposed to mount additional suction blowers in the rear area of the combine in order to draw off the cleaning air stream and feed it to the straw chopper, or to assign the function of the suction blower to the straw chopper, i.e. to feed the cleaning air stream to the straw chopper through separate lines, without using a separate suction blower (cf. EP 1 177 719 A1, which is considered to define the generic type in question).
Finally it has been proposed to connect fans, which are arranged outside of the combine housing and are intended to help discharge the chopped material, to the straw chopper shaft (EP 0 959 659 B1, EP 1 754 408 A2, US 2007/0066370 A1). These fans have no influence on the air pressure in the cleaning unit.
If the air pressure inside the cleaning unit becomes too high, the intended cleaning effect is no longer guaranteed since the flow conditions of the cleaning air stream that were intended by the designer are no longer present. If only the straw chopper is to convey away the cleaning air stream, a large part of its driving power is consumed only for conveying air. The embodiment of EP 1 177 719 A1, in which a separate suction blower is used has the disadvantage that associated drive trains must be provided to drive the blower. In addition, both embodiments require lines, which are difficult to access and therefore difficult to maintain, in the interior of the combine between the cleaning unit and the suction blower or straw chopper. These lines can also disadvantageously influence the air flow inside the combine.
The problem addressed by the invention is considered to be the provision of a combine in which the aforementioned disadvantages are not present or are present only to a reduced extent.